The mirror selfie is the most common outfit photograph there is. It is also a difficult input for a search, and the reasons stack on top of each other in a way that makes the result worse than any one of them would suggest.
All three are fixable, and none of the fixes are difficult.
Why is a reflection softer than a direct photo?
Because of where the camera is focusing and what the mirror does on the way.
The camera focuses on the reflection, which sits at roughly twice the distance from the lens as the mirror surface. So the subject is further away than it appears, and everything in the frame is smaller than it would be in a direct photo taken from the same spot.
Household mirrors are also not optical instruments. They are glass with a coating, and they lose a small amount of contrast and introduce some distortion, particularly towards the edges.
The practical result is that fine detail survives less well. Fabric texture, stitch pattern and small-scale prints are the first things affected, and those are exactly what identify knitwear and patterned garments. A cable knit in a mirror selfie is more likely to read as plain than the same jumper photographed directly, which is the compression problem from finding a knit from a photo with an extra step added.
Where should the phone go?
Low and out from the body, rather than at chest height.
Chest height is where most people naturally hold a phone, and it is the worst place for it. The phone blocks whatever is behind it, and the chest is where most of the identifying detail on a top sits: the neckline, the collar, the placket, any print or graphic, the way the fabric falls from the shoulder.
Held at hip level, tilted upward, the phone covers part of the hip and the waistband instead. That is a real cost on a skirt or a trouser and no cost at all on a top or a coat.
Holding it out from the body rather than against it also stops the phone casting a shadow across the garment, which is a small thing that matters when you are already short on light.
If the outfit's interest is below the waist, hold the phone high instead. The rule is to put it wherever the garment you care about is not.
Should you search the reflection or the direct view?
The reflection, nearly always.
In a mirror photograph you often get both: the reflected front view and, at the edge of the frame, part of the actual person and the back of the phone. Those are two copies of the same outfit at different sizes, sharpness and angles.
The reflection shows the front of the outfit, which is where the identifying detail is. The direct view shows the side of a person holding a phone.
So crop to the reflection. This also solves the multiple-subject problem, since two copies of one person in a frame behaves the same way as two people in a photo: the garment slots get shared between them and each copy gets fewer pixels.
The exception is when the direct view shows something the mirror does not, such as the back of a coat or a bag on the far shoulder. In that case they are two different photographs and worth two separate crops.
What does the room do to the search?
It fills slots you needed for the outfit.
Snagfit returns up to eight garments per photo. A bedroom behind the camera can contain clothes on a rail, a coat on a door, a jumper on a chair and a wardrobe with the door open. All of those are garments, and they get read as garments because that is what they are.
Bathrooms have their own version of the problem: towels are large soft rectangles of textile that can read as fabric, and the light is usually artificial and overhead.
Either way, the fix is the same. Crop to the person and drop the room. That gives the outfit all eight slots and all the resolution, instead of sharing both with a laundry pile.
The general version of this is in how to crop a photo for a better match. The mirror-specific point is that mirror selfies are usually taken in the rooms with the most clothes visible in them, which is not a coincidence and is not helpful.
Does the mirror flip matter?
For anything asymmetric, in a small way.
A mirror reverses left and right. A jacket that buttons one way appears to button the other. A slit on the left leg appears on the right. A logo or any text reads backwards. A bag on one shoulder is on the other.
Snagfit describes what it sees, so an asymmetric detail may be described on the wrong side. In practice this rarely changes which product is found, because retailers do not usually file garments by which side a detail is on. Where it matters is if you are using the photo to describe something to somebody else, or looking at a menswear garment where the button side is a real convention.
Text is the case where it genuinely matters. A slogan or a brand mark in a mirror photo is reversed, and if you were planning to read it, you need to flip the image first. Most phone photo editors will do it in one tap.
What about the flash?
Avoid it if you can, and move instead.
A flash fired at a mirror bounces straight back into the lens. What you get is a blown-out white circle in the middle of the frame, which is normally exactly where the outfit is.
Even where it does not blow out, a direct flash flattens colour and removes shadow. Colour is one of the fields recorded per garment, and shadow is what makes texture readable, so a flash damages both.
If the room is dark enough that you are reaching for the flash, the better move is to move. A window in daylight gives you a light source that is soft, bright and directional, which is the combination that reports colour accurately and keeps texture. The full explanation is in why lighting changes what a search finds.
Does the angle of the mirror change anything?
It changes the proportions, and that affects the shape a search reads.
A phone held below the eyeline and tilted up lengthens the legs and shortens the torso. Held above and tilted down, it does the reverse. Neither is a problem for recognising a garment, and both distort the one field where proportion matters: length.
A skirt photographed from below looks longer than it is. A coat photographed from above looks shorter. Since length is one of the three fields that identify a skirt, that is worth knowing before you decide the results came back wrong.
The fix is to hold the phone at roughly the height of the garment you care about, and to keep the phone parallel to the mirror rather than tilted. That gives you a straight view with no keystone distortion, where vertical lines in the room stay vertical in the picture. It is the same check a photographer uses: if the doorframe behind you leans, the camera is tilted.
What if the mirror is dirty or the light is behind you?
Two common problems with quick fixes.
A dirty or dusty mirror scatters light and lowers contrast across the whole frame, which affects texture the same way compression does. Thirty seconds with a cloth is worth more than any amount of editing afterwards.
A light source behind you, meaning behind the phone, reflects in the mirror and can appear as a bright patch across the outfit. A ceiling light directly overhead does the same thing and adds a shadow under the chin and the chest.
The arrangement that works is light coming from the side, in front of the mirror, with nothing bright directly behind the camera. In practice that usually means standing so a window is to your left or right rather than behind you or in front of you.
Is a deliberate mirror photo worth taking?
Yes, and it is one of the more useful things you can do with a search.
Photographing something you already own is how you find a replacement for a piece that has worn out, a second one in another colour, or something similar now that the original is discontinued.
You control everything: the light, the angle, the background, where the phone is. A deliberate mirror photo taken near a window, with the phone held low, against a plain wall, is a far better input than a casual one taken in a dim bathroom.
Better still, take the garment off and photograph it flat or on a hanger against a plain wall. That removes the mirror, the phone and the room in one move, and it is close to how a retailer photographs stock. The whole approach is covered in shopping your camera roll as one of the uses people overlook.